The fourth and final volume of the Intergovernmental Panel on Climate Change's giant climate assessment offered no surprises, nor was it expected to since it combined the findings of three reports released in the past 13 months.

But it underlined the scope of the climate challenge in stark terms. Emissions, mainly from the burning of fossil fuels, may need to drop to zero by the end of this century for the world to have a decent chance of keeping the temperature rise below a level that many consider dangerous.

The IPCC didn't say exactly what such a world would look like but it would likely require a massive shift to renewable sources to power homes, cars and industries combined with new technologies to suck greenhouse gases from the atmosphere.

The report warned that failure to reduce emissions could lock the world on a trajectory with "irreversible" impacts on people and the environment. Some impacts already being observed included rising sea levels, a warmer and more acidic ocean, melting glaciers and Arctic sea ice and more frequent and intense heat waves.

"Science has spoken. There is no ambiguity in their message. Leaders must act. Time is not on our side," UN Secretary-General Ban Ki-moon said at the report's launch in Copenhagen.

Amid its grim projections, the report said the tools are there to set the world on a low-emissions path and break the addiction to burning oil, coal and gas which pollute the atmosphere with heat-trapping CO2, the chief greenhouse gas.

"All we need is the will to change, which we trust will be motivated by knowledge and an understanding of the science of climate change," IPCC chairman Rajendra Pachauri said.

The IPCC was set up in 1988 to assess global warming and its impacts. The report released Sunday caps its latest assessment, a mega-review of 30,000 climate change studies that establishes with 95-per-cent certainty that most of the warming seen since the 1950s is man-made. The IPCC's best estimate is that just about all of it is man-made, but it can't say that with the same degree of certainty.

Today only a small minority of scientists challenge the mainstream conclusion that climate change is linked to human activity.

Global Climate Change, a NASA website, says 97 per cent of climate scientists agree that warming trends over the past century are very likely due to human activities.

The American public isn't as convinced. A year-old survey by Pew Research showed 67 per cent of Americans believed global warming is occurring and 44 per cent said the earth is warming mostly because of human activity. More recently, a New York Times poll said 42 per cent of Republicans say global warming won't have a serious impact, a view held by 12 per cent of Democrats and 22 per cent of independents.

Sleep-deprived delegates approved the final documents Saturday after a weeklong line-by-line review that underscored that the IPCC process is not just about science. The reports must be approved both by scientists and governments, which means political issues from UN climate negotiations, which are nearing a 2015 deadline for a global agreement, inevitably affect the outcome.

The rift between developed and developing countries in the UN talks opened up in Copenhagen over a passage on what levels of warming could be considered dangerous. After a protracted battle, the text was dropped from a key summary for policy-makers -- to the disappointment of some scientists.

"If the governments are going to expect the IPCC to do their job," said Princeton professor Michael Oppenheimer, a lead author of the IPCC's second report, they shouldn't "get caught up in fights that have nothing to do with the IPCC."

The omission meant the word "dangerous" disappeared from the summary altogether. It appeared only twice in a longer underlying report compared to seven times in a draft produced before the Copenhagen session. The less loaded word "risk" was mentioned 65 times in the final 40-page summary.

"Rising rates and magnitudes of warming and other changes in the climate system, accompanied by ocean acidification, increase the risk of severe, pervasive, and in some cases irreversible detrimental impacts," the report said.

World governments in 2009 set a goal of keeping the temperature rise below 2 degrees C compared to before the industrial revolution. Temperatures have gone up about 0.8 C since the 19th century.

Emissions have risen so fast in recent years that the world has used up two-thirds of its carbon budget, the maximum amount of CO2 that can be emitted to have a likely chance of avoiding 2 degrees of warming, the IPCC report said.

"This report makes it clear that if you are serious about the 2-degree goal ... there is nowhere to hide," said Alden Meyer of the Union of Concerned Scientists, an advocacy group. "You can't wait several decades to address this issue."

U.S. Secretary of State John Kerry said the report demands "ambitious, decisive and immediate action."

"Those who choose to ignore or dispute the science so clearly laid out in this report do so at great risk for all of us and for our kids and grandkids," Kerry said in a statement.

The IPCC said the cost of actions such as shifting to solar and wind power and other renewable sources and improving energy efficiency would reduce economic growth only by 0.06 per cent annually.

Pachauri said that should be measured against the implications of doing nothing, putting "all species that live on this planet" at peril.

The report is meant as a scientific roadmap for the UN climate negotiations, which continue next month in Lima, Peru. That's the last major conference before a summit in Paris next year, where a global agreement on climate action is supposed to be adopted.

The biggest hurdle is deciding who should do what. Rich countries are calling on China and other major developing countries to set ambitious targets; developing countries saying the rich have a historical responsibility to lead the fight against warming and to help poorer nations cope with its impacts. The IPCC avoided taking sides, saying the risks of climate change "are generally greater for disadvantaged people and communities in countries at all levels of development."